**Core Concept**
Psoriasis is a chronic inflammatory skin disease characterized by abnormal keratinocyte proliferation and immune cell infiltration. Systemic corticosteroids can provide rapid control of symptoms but are not a first-line treatment due to potential side effects. The use of systemic steroids in psoriasis is generally limited to specific situations.

**Why the Correct Answer is Right**
The definitive indication of systemic steroids in psoriasis is **severe, life-threatening generalized pustular psoriasis**. This condition is a medical emergency characterized by widespread pustules, fever, and systemic inflammation. Systemic steroids are the mainstay of treatment for this condition, helping to reduce inflammation and prevent complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Severe plaque psoriasis is not an indication for systemic steroids, as other treatments like biologics and phototherapy are generally more effective and safer.

**Option B:** Psoriatic arthritis is treated with systemic steroids, but they are not the definitive indication. Other treatments like NSAIDs, DMARDs, and biologics are often used first.

**Option C:** Erythrodermic psoriasis is a severe form of psoriasis, but systemic steroids are not the definitive treatment. Other treatments like phototherapy and biologics are often used first.

**Clinical Pearl / High-Yield Fact**
In severe, life-threatening generalized pustular psoriasis, systemic steroids should be started immediately, but their use should be brief due to the high risk of rebound and worsening of symptoms upon withdrawal.

**Correct Answer:** C.